Connected: An Internet Encyclopedia
RFC 1583 - 16.4. Calculating AS external routes

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### 16.4. Calculating AS external routes

AS external routes are calculated by examining AS external link advertisements. Each of the AS external link advertisements is considered in turn. Most AS external link advertisements describe routes to specific IP destinations. An AS external link advertisement can also describe a default route for the Autonomous System (Destination ID = DefaultDestination, network/subnet mask = 0x00000000). For each AS external link advertisement:

1. If the cost specified by the advertisement is LSInfinity, or if the advertisement's LS age is equal to MaxAge, then examine the next advertisement.

2. If the advertisement was originated by the calculating router itself, examine the next advertisement.

3. Call the destination described by the advertisement N. N's address is obtained by masking the advertisement's Link State ID with the network/subnet mask contained in the body of the advertisement. Look up the routing table entry for the AS boundary router (ASBR) that originated the advertisement. If no entry exists for router ASBR (i.e., ASBR is unreachable), do nothing with this advertisement and consider the next in the list. Else, this advertisement describes an AS external path to destination N. Examine the forwarding address specified in the AS external link advertisement. This indicates the IP address to which packets for the destination should be forwarded. If the forwarding address is set to 0.0.0.0, packets should be sent to the ASBR itself. Otherwise, look up the forwarding address in the routing table.[23] An intra-area or inter-area path must exist to the forwarding address. If no such path exists, do nothing with the advertisement and consider the next in the list.

   Call the routing table distance to the forwarding address X (when the forwarding address is set to 0.0.0.0, this is the distance to the ASBR itself), and the cost specified in the advertisement Y. X is in terms of the link state metric, and Y is a type 1 or 2 external metric.

4. Next, look up the routing table entry for the destination N. If no entry exists for N, install the AS external path to N, with next hop equal to the list of next hops to the forwarding address, and advertising router equal to ASBR. If the external metric type is 1, then the path-type is set to type 1 external and the cost is equal to X+Y. If the external metric type is 2, the path-type is set to type 2 external, the link state component of the route's cost is X, and the type 2 cost is Y.

5. Else, if the paths present in the table are not type 1 or type 2 external paths, do nothing (AS external paths have the lowest priority).

6. Otherwise, compare the cost of this new AS external path to the ones present in the table. Type 1 external paths are always shorter than type 2 external paths. Type 1 external paths are compared by looking at the sum of the distance to the forwarding address and the advertised type 1 metric (X+Y). Type 2 external paths are compared by looking at the advertised type 2 metrics, and then if necessary, the distance to the forwarding addresses.

   If the new path is shorter, it replaces the present paths in the routing table entry. If the new path is the same cost, it is added to the routing table entry's list of paths.

Next: 16.5. Incremental updates -- summary link advertisements

Connected: An Internet Encyclopedia
RFC 1583 - 16.4. Calculating AS external routes